I hope everyone know that by visiting www.whatismyip.com will enable us to determine our own ip. I am not sure why each time my ip address changes every time I go online. Can anybody answer my questions? Is it something to do with my ISP?

Yes your ISP runs on a dynamic ip addressing system (i think thats how it goes)
but anyways your ip address is dynamic... meaning it changes everytime..
you probably are using a dialup connection...

Quote:

IP address: The Internet Protocol (IP) address is assigned to your computer, providing a numeric address necessary to connect to the Internet or another IP-based network.

Static IP: A static IP address is fixed, much like a telephone number. If your ISP gives you a static address, you will always use the same address. Servers usually have static addresses, so they can always be found at the same location.

Dynamic IP: A dynamic IP address changes each time you connect to your Internet Service Provider (ISP). This allows ISPs to keep a pool of addresses available to subscribers. If you disconnect from the ISP, your address is returned to the pool, becoming available to the next computer that connects.

I have found that with cable, although NTL do use DHCP with a 2 week lease, if your computer / gateway / router is on 24/7 you will not lose your IP

I noticed that as well, i think i have had my IP address about 6 months... Also noticed it is only when you attach another PC to the STB that you are assigned a new IP, but as soon as you put the old PC back you are with your previous IP.

I think it is all done (NTL) by MAC addresses of the network cards. Its pretty good though, as a longer lease allows you more opportunity to host your own servers etc.

It is worth mentioning (if anyone is interested) that some cable companies and some ISPs (like aol) used to have IP sharing set up where same IP address is shared between more than 1 users at the same time. This way they could save a lot on network cost and bandwidth but would slow down the connection for the individual users sharing the same IP.

They were supposted to stop this practice after complains but, I wouldn't bet on it.

As others have mentioned. with some of the 'always on' ISPs (ie Cable and ADSL), in theory, you should always keep the same IP address.

This is how DHCP works.

When the IP address lease expires, the DHCP server will try and give you the same address by default. If however you disconnect for some time, there is always the chance that that address will get allocated to somebody else in the meantime in which case, you will be given the next available address instead.

There is however a shortage of IP addresses and RIPE (in Europe) frown upon the idea of giving out public IP addresses for the hell of it.

Therefore more and more ISPs are using NAT in their network so that their end users are not given a public IP address. Under normal usage, this does not cause a problem and enables the ISP to have more concurrent users than it has public IP addresses.

Indeed AOL is a classic example of all the above.

It has been known for an AOL user to have a different IP address every few seconds. I have seen more than a few ecommerce outfits fall foul of this where they have used the users IP address to track users and map them to say a shopping cart.

In this case, the use of cookies provides a way of keeping track (see they do have benefits to them).

If you were to tell us who your ISP is, we can probably tell you the reason for your changing IP address but more than likely, it is going to be one of the above reasons.